1. Field of the Invention
This invention relates to a method for the preparation of halide, preferably fluoride, glass articles, e.g., preforms suitable for drawing into fibre.
2. Related Art
Halide, and especially fluoride, glass fibre is used where transmission in the wavelength band 2000 nm to 4500 nm is required. Halide fibres also display good transmission outside this band, e.g., over the range 500 nm to 2000 nm but competitors, e.g., SiO.sub.2 based fibres, have good transmission in this region. However, the competitors have such high attenuations in the band 2000 nm to 4500 nm that they are excluded for consideration when it is required to use the fibre at this wavelength.
In most cases, the preparation of halide fibres involves first the preparation of the core and clad glasses, the casting of these two glasses to make a preform and the drawing of the preform into fibre. It is important to avoid contamination during the preparation of the glasses and their casting. For this reason it is usual to carry out these stages in isolation chambers which are provided with a dry inert atmosphere at slightly above the pressure outside the isolation chamber. The inert atmosphere is usually nitrogen for reason of cheapness but other inert gases, e.g., argon or helium, could also be used. It is also advantageous to submit the melt to an oxidation process and mixtures of inert gas and oxygen are used for this purpose. The transmission properties of a halide fibre are determined to a large extent by chemical considerations, e.g., the chemical composition of the core glass and the clad glass. It is also important that the two glass compositions cooperate to provide guidance and are compatible with one another during the preparative stages, especially the drawing.
The selection of the chemical compositions of the core and clad glasses together with the careful preparative techniques indicated above are important to achieve low attenuation but it appears that mechanical imperfections, e.g., crystals and bubbles, in the fibre can also cause attenuation, probably because mechanical imperfections can scatter the light.